Earlier quoted context omitted.
By default, you get emails in FB groups for each message and comment, just like Posterous, and you can respond via email to post new messages (each group has its own email address) or comments. You could interact with multiple groups without ever logging into FB (I know because I do :). I'm not sure about the media thing, though it's not really practical for video to attach the media...surely posterous doesn't do tha…
It's the little things that make all the difference imo. For example, while Facebook Groups does allow commenting by email, if you attach a picture to a reply, it doesn't appear in the group. Secondly, I don't see many small businesses or even teams within large companies using Facebook Groups to collaborate.
Not sure if that's relevant, since I don't see any way that they start using Posterous for that.
Whatever, I'm just throwing out my opinion that this doesn't seem like a big deal in Dec 2010. Maybe the ability to attach inline pictures will make all the difference, though.